G-protein coupled receptors (GPCRs) are one of the major class of proteins that are responsible for transducing a signal within a cell. GPCRs are proteins that have seven transmembrane domains. Upon binding of a ligand to the extracellular domain of a GPCR, a signal is transduced within the cell which results in a change in a biological or physiological property of the cell.
GPCRs, along with G-proteins and effectors (intracellular enzymes and channels which are modulated by G-proteins), are the components of a modular signaling system that connects the state of intracellular second messengers to extracellular inputs. These genes and gene-products are potential causative agents of disease (Spiegel et al. (1993) J. Clin. Invest. 92:1119-1125; McKusick and Amberger (1993) J. Med. Genet. 30:1-26). Specific defects in the rhodopsin gene and the V2 vasopressin receptor gene have been shown to cause various forms of autosomal dominant and autosomal recessive retinitis pigmentosa (see Nathans et al. (1992) Annu. Rev. Genet. 26:403-424), nephrogenic diabetes insipidus (Holtzman et al. (1993) Hum. Mol. Genet. 2:1201-1204 and references therein). These receptors are of critical importance to both the central nervous system and peripheral physiological processes. Evolutionary analyses suggest that the ancestor of these proteins originally developed in concert with complex body plans and nervous systems.
The GPCR protein superfamily now contains over 250 types of paralogues, receptors that represent variants generated by gene duplications (or other processes), as opposed to orthologues, the same receptor from different species. The superfamily can be broken down into five families: Family I, receptors typified by rhodopsin and the beta2-adrenergic receptor and currently represented by over 200 unique members (reviewed by Dohlman et al. (1991) Annu. Rev. Biochem. 60:653-688 and references therein); Family II, the recently characterized parathyroid hormone/calcitonin/secretin receptor family (Juppner et al. (1991) Science 254:1024-1026; Lin et al. (1991) Science 254:1022-1024); Family III, the metabotropic glutamate receptor family in mammals (Nakanishi (1992) Science 258:597-603); Family IV, the cAMP receptor family, important in the chemotaxis and development of D. discoideum (Klein et al. (1988) Science 241:1467-1472); and Family V, the fingal mating pheromone receptors such as STE2 (reviewed by Kurjan (1992) Annu. Rev. Biochem. 61:1097-1129).
In addition to these groups of GPCRs, there are a small number of other proteins which present seven putative hydrophobic segments and appear to be unrelated to GPCRs; however, they have not been shown to couple to G-proteins. Drosophila express a photoreceptor-specific protein bride of sevenless (boss), a seven-transmembrane-segment protein which has been extensively studied and does not show evidence of being a GPCR (Hart et al. (1993) Proc. Natl. Acad. Sci. USA 90:5047-5051 (1993)). The gene frizzled (fz) in Drosophila is also thought to be a protein with seven transmembrane segments. Like boss, fz has not been shown to couple to G-proteins (Vinson et al. (1989) Nature 338:263-264).
G proteins represent a family of heterotrimeric proteins composed of .alpha., .beta. and .gamma. subunits, which bind guanine nucleotides. These proteins are usually linked to cell surface receptors, e.g., receptors containing seven transmembrane domains, such as the ligand receptors. Following ligand binding to the receptor, a conformational change is transmitted to the G protein, which causes the .alpha.-subunit to exchange a bound GDP molecule for a GTP molecule and to dissociate from the .beta..gamma.-subunits. The GTP-bound form of the .alpha.-subunit typically functions as an effector-modulating moiety, leading to the production of second messengers, such as cyclic AMP (e.g., by activation of adenylate cyclase), diacylglycerol or inositol phosphates. Greater than 20 different types of .alpha.-subunits are known in man, which associate with a smaller pool of .beta. and .gamma. subunits. Examples of mammalian G proteins include Gi, Go, Gq, Gs and Gt. G proteins are described extensively in Lodish H. et al. Molecular Cell Biology, (Scientific American Books Inc., New York, N.Y., 1995), the contents of which are incorporated herein by reference.
GPCRs are a major target for drug action and development. Accordingly, it is valuable to the field of pharmaceutical development to identify and characterize previously unknown GPCRs. The present invention advances the state of the art by providing a previously unidentified GPCR which is expressed predominantly in the brain.